1. Field of the Invention
The invention relates generally to the field of disc drives, and more particularly to an apparatus and method for detecting air contamination of fluid dynamic bearings within a disc drive.
2. Description of the Related Art
Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism.
Generally, the discs are mounted on a hub that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft supported from a base plate of the housing. Permanent magnets attached to the hub interact with a stator winding to rotate the hub. One or more bearings usually support the hub for rotation.
Over time, disc drive storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping.
One alternative bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing (i.e., shaft) and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids (i.e., hydrodynamic fluid). Fluid dynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run-out.
Generally, during the manufacturing of the fluid dynamic bearings using oil or ferromagnetic fluids, the fluid dynamic bearing undergoes a lubricating fluid fill process. During the lubricating fluid fill process, air is inadvertently introduced into the lubricating fluid in the form of bubbles. Unfortunately, the bubbles may cause fluid pressure inconsistencies within the fluid dynamic bearing. Further, during operation, the air bubbles may expand increasing non-repeatable run-out between the rotating and fixed members.
Generally, for non-fluid dynamic bearings, such as stationary shaft and two piece hub-shaft motors, the meniscus of the fluid is checked under a vacuum for changes due to air contamination. That is, a change typically indicates the presence of a bubble, which expands. For example, a microscope may be used to visually check the fluid meniscus change in dimension when a vacuum is applied. If air is present in the non-fluid dynamic bearings, the meniscus width, height, etc., within the capillary may vary as a function of the amount of air present.
Generally, differential weight changes before and after the fill process are used to inspect the air contamination of fluid within a fluid dynamic bearing. Unfortunately, this methodology is time consuming and prone to measurement variation as the amount of air within the hydrodynamic fluid may be very small. Accordingly, the measurements may lead to an increase in disc drive manufacturing time, premature disc drive failure due to inaccurate measurements, and ultimately an increase in the cost of the disc drive.
Therefore, a need exists for a method and apparatus to provide a reliable and repeatable fluid dynamic bearing air-bubble contamination test.